In the industrial production of spun yarns it is common practice for them to be collected on an idle tube carried by a bobbin-carrying arm, which rests on a rotating driving roller and takes up the spun yarn coming from a feed element to wind it onto itself. The bobbin is thus formed by pulling and winding the spun yarn on its surface, it being drawn in rotation by the roller underneath on which the bobbin being formed rests. This practice allows the spun yarn to be wound at a substantially constant linear speed, irrespective of the increasing dimensions of the bobbin and depending only on the rotation speed of said driving roller. The spun yarn is wound in spirals onto the rotating bobbin as the pick-up unit is provided with a thread-guiding device which distributes the spun yarn on the outer surface of the bobbin with backward and forward axial motion. In industry, the bobbins may be shaped like a truncated cone or a straight cylinder with substantially flat bases, with the exception of a few specific cases in which the terminal parts of the bobbins are shaped with a pronounced flare.
In the prevalent industrial use of spun yarn in bobbins, downstream working requires the bobbin to be conical in shape, for example when the spun yarn is unwound in an axial direction from the bobbin fixed on creels. This conicity is however slight and restricted to a few degrees of inclination of the generatrix of the cone in relation to its axis, generally between 2° and 6°, except for some specific uses for which “superconic” bobbins are required.
In the case of winding on a winder the most widespread device for distribution of the spun yarn on the surface of the bobbin with axial backward and forward motion consists of a spiral backward and forward groove cut into the surface of the driving roller which causes the spun yarn to perform an axial excursion of a pre-established length, for a pre-established number of turns of the roller and with a pre-established wind ratio. In other words, the yarn winding and spun yarn distribution elements operate according to a fixed speed ratio.
However, in other cases the device for distribution of the yarn on the bobbin is produced with an independent thread-guiding device, moved by its own driving element, with which the frequency of the backward and forward movement, its travel, the length of the spiral wound and the wind ratio, etc. may be modulated time by time and according to need.
Typically, distribution of the spun yarn on the bobbin with modulatable thread-guide is required in open-end spinning frames, for which distribution of the spun yarn on the bobbin with grooved cylinder does not meet the conditions required for efficacious winding on a bobbin of the desired quality. These winding conditions in particular include its wind ratio, the speed and excursion travel, which cannot be maintained at a single pre-established value, as is the case with the grooved cylinder, but must be adapted time by time to the spun yarn being produced and also modulated during production of the bobbin. There are also other impediments to the use of the grooved cylinder, both due to the geometry of the system and to the overall open-end spinning procedure.
In open-end spinning there is a further limiting condition in that the spun yarn is produced at constant linear speed and therefore must be picked up at a speed corresponding to the speed at which it is made available, substantially equal and constant, maintaining it at a moderate tension, while when forming both straight cylindrical and conical bobbins the pick-up speed typically has a pulsating trend.
It must also be borne in mind that, to compensate these pulsations of adjusting tension and path length, the elasticity of the spun yarn could be taken into account only within the limits of a few percent, also because the yarn is already stressed considerably at the operating speed of current open-end spinning frames.
To explain more clearly the problems dealt with and the technical solutions proposed with the present invention reference is made, in the description below, to pick-up of “open-end” spun yarns on bobbins, provided purely as a non-limiting example, it being explicitly specified that it may be used advantageously to wind spun yarns produced with different spinning technologies on bobbins.